Method and apparatus for fabricating nanoscale structures

ABSTRACT

An apparatus comprises a scanning electron microscope (SEM) ( 1 ) positioned over a manipulation chamber ( 2 ) which houses a sample holder ( 3 ). The walls of the manipulation chamber ( 2 ) support two probes ( 4, 4   a ) and the sample holder ( 3 ) is able to hold a sample ( 5 ), such as carbon nanotubes ( 10   a ) carried on a substrate ( 10 ). The apparatus can selectively move and apply voltages and currents to the probe or probes ( 4, 4   a ) and sample holder ( 3 ) under the SEM ( 1 ). By controlling the current that is passed across a contact between the probe ( 4 ) and a carbon nanotube ( 10   a ), a conditioned weld is formed. Likewise, by controlling the current that is passed along a carbon nanotube ( 10   a ), the nanotube ( 10   a ) can be annealed. Using both the probes ( 4, 4   a ) a carbon nanotube can be held and cut at any position along its length. This allows the formation of novel carbon nanotube structures.

FIELD OF THE INVENTION

This invention relates to a method and apparatus for fabricatingnanoscale structures. More specifically, the invention concerns a methodof welding a nanoscale wire to a structure, a method of annealing ananoscale wire and a method of cutting a nanoscale wire, along withapparatus for carrying out the methods and the nanoscale structures thatcan be produced by the methods.

BACKGROUND OF THE INVENTION

The potential for nanoscale structures to be fabricated from nanoscalewires is being very actively researched. Nanoscale wires and, inparticular, carbon nanotubes, have interesting properties and thepotential to form a vast array of nanoscale electromechanical devices.For example, the small size (down to diameters of a few nanometres);ability to tolerate high electric current density; and semi-conductingor metallic electrical characteristics of carbon nanotubes make themideal candidates as key elements in the next generation of electronicdevices. However, carbon nanotubes are presently grown in bulk, eitheron substrates or as tangled bundles. This imposes severe limitations onthe fabrication of specific devices or structures from carbon nanotubes.Consequently, a significant proportion of research into these materialshas concentrated on applications suited to these production methods,such as their use for reinforcing materials; providing embeddedconductive fibres in polymers; or their use in field emission tip arraysfor flat panel displays.

The ability to position individual carbon nanotubes at chosen locationsand selectively create electronically reliable nanotube to nanotube, ornanotube to substrate junctions has not yet been demonstrated. Thelimited number of electronic devices formed from carbon nanotubes up tonow largely rely on scattering many nanotubes onto suitable substrates,followed by laying down conventional electrical contacts and thensearching for the correct combination and/or orientation of nanotubes toconstitute a rudimentary device. So, there is a need to develop ways fornanotube devices to be fabricated in a controlled and selective mannerand to be manipulated to produce custom built electronic devices. Thismay enable them to become the basis of future electronic devices. At thesame time, it is desirable for this to be made possible using fibresgrown using existing growth techniques.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provideda method of welding a nanoscale wire to a structure, the methodcomprising:

positioning the nanoscale wire and the structure in contact with oneanother; and

applying a voltage across the contact so that a current flows throughthe contact and heats it to weld the wire to the structure.

According to a second aspect of the present invention, there is providedan apparatus for welding a nanoscale wire to a structure, the apparatuscomprising:

a manipulator for positioning the nanoscale wire and the structure incontact with one another, and

a controller for applying a voltage across the contact so that a currentflows though the contact and heats it to weld the wire to the structure.

So, a nanoscale wire, such as a carbon nanotube, and another structure,such as the probe of a manipulator, can be brought into contact with oneanother and, by passing a current between the wire and the structure,through the contact, a weld can be formed. The applicants haverecognised that, usually, the electrical resistance of the contact isinitially higher than the resistance of the wire or the other structure.Thus, when a current is passed across the contact between the wire andthe structure, the contact is heated by the current more than the wireor the other structure and a weld is formed.

The invention allows a weld to be formed without damage to the wire orthe other structure. However, in order to reduce the risk of the wire orthe other structure being damaged during welding, it is preferred tolimit the current that flows through the contact during welding. Inother words, the controller preferably limits the current that flowsthrough the contact during welding. Indeed, the current may be limitedto below a welding current threshold. This is typically set lower thanthe typical current that can be carried by the particular type ofnanoscale wire being welded before it overheats and either fails or isstructurally damaged. This can be established by experiment. Usually,the welding current limit is in the order of 10 μA, although thisdepends greatly on the type of wire.

A voltage of less than around 5 V is usually sufficient to generate therequired current. In one example, the voltage can be applied across thecontact just once. Similarly, the current may be held steady for apredetermined period of time, e.g. between around is and around 100 s.This might be useful when experiments have established the current andduration required to obtain an optimum weld. However, it is preferredthat a voltage is applied across the contact more than once. In otherwords, a voltage may be applied across the contact during pluralseparate intervals. So, the apparatus may comprise a controller forapplying a voltage across the contact during plural separate intervals.

The applicants have recognised that this repeated application of thevoltage conditions the weld and allows its quality to be monitoredduring formation. More specifically, by repeatedly applying a knownvoltage or voltage wave-form across the contact and measuring thecurrent in successive applications, it is possible to detect reductionsin the resistance of the contact. Reducing resistance can be indicativeof improved electrical and mechanical properties of the weld.Furthermore, the applicants have recognised that when the resistancestops falling, the weld has reached optimum quality.

So, it is preferred that the method comprises monitoring the currentpassing through the contact while the voltage is applied. In otherwords, it is preferred that the controller monitors the current passingthrough the contact while the voltage is applied. It is also preferredthat the method comprises comparing the current when a known voltage isapplied with the current at that voltage when it is applied again. Inother words, it is preferred that the controller compares the currentwhen a known voltage is applied with the current at that voltage when itis applied again. Thus the change in resistance of the contact can bemonitored. The comparison may be between voltages applied duringdifferent intervals, e.g. between succeeding applications of thevoltage. However, it is preferred that the voltage is increased anddecreased during each individual interval and the current at a voltageduring the increase is compared with the current at that voltage duringthe decrease. To improve accuracy, the current can be compared at pluralrespective voltages or a voltage-current relationship can be compared.

As mentioned above, when no further substantial fall in resistance isdetected, the weld can be considered to be optimum. It is thereforepreferred the method comprises continuing to apply the voltage acrossthe contact (e.g. applying the voltage during another interval) untilthe comparison shows that there is no substantial difference in current.In other words, the apparatus may comprise the controller continuing toapply a voltage across the contact (e.g. applying the voltage duringanother interval) until the comparator shows that there is nosubstantial difference in current. This might be when the difference incurrent is less than a pre-set limit, e.g. 1%.

As mentioned above, the other structure might typically be a probe formanipulating a nanoscale wire, e.g. a nanoscale probe. However, theother structure can be a variety of other devices or components. Forexample, the other structure may be a substrate for a nanoscale wire.Alternatively, it may be another nanoscale wire. So, the ability of theinvention to weld nanoscale wires to a variety of other structures,including other nanoscale wires, and condition the welds to formoptimised electrical and mechanical connections, allows a large numberof new nanoscale structures to be formed. According to a third aspect ofthe present invention, there is therefore provided a nanoscale structureproduced using the above methods. These structures can take a variety ofdifferent forms, but are characterised by including one or more weldsformed using the above methods.

When the other structure is a moveable probe, once the nanoscale wirehas been welded to the probe, the probe can be moved to move the wire orto exert strain on it. As the weld is mechanically strengthened by theconditioning, relatively large forces can be applied by the probewithout the weld breaking. Furthermore, as the weld has good electricalcharacteristics, relatively large currents can be passed through thenanoscale wire. The applicants have recognised that this can allow theelectrical and mechanical characteristics of the nanoscale wire itselfto be improved by annealing. In other words, the structure may be aprobe and the method may comprise passing a current along the wire viathe probe sufficient to heat the wire and cause annealing. Similarly,the structure may be a probe and the controller may pass current alongthe wire via the probe sufficient to heat the wire and cause annealing.

The applicants believe this to be new in itself and, according to afourth aspect of the present invention, there is provided a method ofannealing a nanoscale wire, the method comprising welding a probe to thewire and passing current along the wire via the probe sufficient to heatthe wire and cause annealing.

Likewise, according to a fifth aspect of the present invention, there isprovided an apparatus for annealing a nanoscale wire, the apparatuscomprising means for welding a probe to the wire and a controller forpassing a current along the wire via the probe sufficient to heat thewire and cause annealing.

So, simply heating the wire can anneal it and improve its electrical andmechanical characteristics. However, moving the probe can exert strainon the wire to straighten or bend the wire during annealing. It istherefore preferred that the method includes moving the probe to exertstrain on the wire. The probe may exert strain on the wire by bendingthe wire. Alternatively, the probe may exert strain on the wire bystraightening the wire.

Once the nanoscale wire has been welded and/or conditioned, it may bedesirable to cut it. For example, the nanoscale wire may be attached toa substrate from which it is desirable to free it. It is thereforepreferred that the method further comprises:

positioning a cutting probe at a position along the length of the wireintermediate two positions at which the wire is held; and

applying an electrical potential between the cutting probe and the wireto cut the wire at the position along the length of the wire.

Likewise, it is preferred that the apparatus further comprises amanipulator for positioning a cutting probe at a position along thelength of the wire intermediate two positions at which the wire is heldand that the controller applies an electrical potential between thecutting probe and the wire to cut the wire at the position along thelength of the wire.

The applicants believe this to be new in itself and, according to asixth aspect of the present invention, there is therefore provided amethod of cutting a nanoscale wire, the method comprising:

positioning a cutting probe at a position along the length of the wireintermediate two positions at which the wire is held; and

applying an electrical potential between the cutting probe and the wireto cut the wire at the position along the length of the wire.

Likewise, according to a seventh aspect of the present invention, thereis provided an apparatus for cutting a nanoscale wire, the apparatuscomprising:

a manipulator for positioning a cutting probe at a position along thelength of the wire intermediate two positions at which the wire is held;and

a controller for applying an electrical potential between the cuttingprobe and the wire to cut the wire at the position along the length ofthe wire.

One of the two positions might be the position at which the wire iswelded to the structure. The other of the two positions might be thepoint at which the wire contacts a substrate, e.g. on which it wasgrown. Typically, the cutting probe is positioned to touch the wire atthe position along the length of the wire and the electrical potentialis applied only between the cutting probe and one of the two positionsat which the wire is held. This results in an electric current flowingonly in a portion of the wire between the position that the cuttingprobe touches the wire and the one of the two positions. So, only thatportion of the wire is heated and cut away from the remaining portion.To achieve this, the current is typically relatively high. For example,the applied potential can be controlled to pass a current exceeding theestimated typical current at which the nanoscale wire fails or isstructurally damaged.

Alternatively, the cutting probe can be positioned so that it is closestto the wire at the position along the length of the wire, but slightlyspaced away from the wire. When the electric potential is applied, thewire is then vaporised at the position along the length of the wire bythe electric field between the wire and the probe. In this case, it ispreferred that the applied electrical potential is alternated.

The ability of the invention to weld, anneal and cut nanoscale wiresallows a large number of new nanoscale structures to be formed.According to an eighth aspect of the present invention, there istherefore provided a nanoscale structure produced using any of the abovemethods.

The methods of the present invention may be implemented at leastpartially using software e.g. computer programs. According to furtheraspects of the present invention, there is therefore provided computersoftware specifically adapted to carry out the methods described abovewhen installed on a computer. The invention also extends to a computersoftware carrier comprising such software. The computer software carriercould be a physical storage medium such as a ROM chip, CD ROM or disk,or could be a signal such as an electronic signal over wires, an opticalsignal or a radio signal such as to a satellite or the like.

Nanoscale is intended to mean having at least one dimension measuringbetween 1 nm and 1 μm. For example, the diameter of a nanoscale wiremight be between 1 nm and 1 μm. Indeed, it is preferred that thenanoscale wire(s) mentioned above are carbon nanotube(s) and thesetypically have diameters up to around 100 nm. However, the invention isnot limited to carbon nanotubes. Rather, the nanoscale wire(s) may benanofibre(s), nano-powder(s), nano-particle(s), nano-rod(s),nano-structure(s), carbon sphere(s) and single crystal nanowire(s).Likewise, the wire may be on a larger micron or millimetre scale. Whilstthese nanoscale wire(s) and such like should be conductive, they may beinorganic or organic. Examples of suitable inorganic materials might becarbon or silicon. Organic materials might include conductive polymersor protein based fibres such as DNA, enzymes or micro channels.

Reference to carbon nanotubes is not limited to carbon nanotubesproduced by any particular method, and as such, nanotubes produced byany recognised method described in the literature can be manipulated bythe methods of the invention. It should also be understood that thecarbon nanotubes referred to in this specification may be either singlewall or multi-wall nanotubes; that is they may be considered to beconstructed from one or more concentric layers of graphitic carbonmaterial. They may also be Silicon nanowires or any other nano/microwire composed of inorganic conducting material.

Preferred embodiments of the invention are now described, by way ofexample only, with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an apparatus according to thepresent invention;

FIG. 2 is a schematic illustration of a method of welding a carbonnanotube to a probe using the apparatus of FIG. 1;

FIG. 3 is a loglinear graph of current versus voltage during welding;

FIG. 4 is a schematic illustration of a method of cutting a carbonnanotube using the apparatus of FIG. 1; and

FIG. 5 is a schematic illustration of a method of welding a carbonnanotube to another carbon nanotube using the apparatus of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, an apparatus comprises a scanning electronmicroscope (SEM) 1 positioned over a manipulation chamber 2 (or SEMchamber) which houses a sample holder 3 (or SEM stage). The walls of themanipulation chamber 2 support two probes 4, 4 a and the sample holder 3is able to hold a sample 5, such as carbon nanotubes 10 a carried on asubstrate 10 or arranged on a support. In other embodiments, more thantwo probes 4, 4 a are provided and the probes 4, 4 a are supported onthe sample holder 3 (or SEM stage).

In this embodiment, the probes 4, 4 a each comprise sharp implements ormanipulators having tip radius in the range around 5 nm to around 100μm. In other embodiments, the probes 4, 4 a are hook-shaped. Theelectrical, physical and mechanical properties of tungsten make it aparticularly suitable material for the probes 4, 4 a. However, theprobes 4, 4 a can be made from metals other than tungsten. Indeed, theycan be made from any electrically conducting material. Alternatively,they can be oxide-coated or semi-conducting to allow more extensiveevaluation of the electrical properties of the nanotubes.

The probes 4, 4 a are electrically isolated from the manipulationchamber 2, each other and sample holder 3, but connected to externalwires 6, 6 a passing through the wall of the manipulation chamber 2.Likewise, the sample holder 3 is arranged to electrically isolate thesample 5 from the manipulation chamber 2 and the probes 4, 4 a, butconnect it to an external wire 7 passing through the wall of themanipulation chamber 2. The purpose of the electrical connections is toallow electric potential to be applied to the probes 4, 4 a and thesample holder 3; and to allow electric current to be passed throughcircuits formed between the probes 4, 4 a and the sample holder 3, e.g.via the sample 5.

For this purpose, a power supply 8 is connected to the external wires 6,6 a, 7. The power supply 8 is capable of selectively applying electricpotential between any combination of wires 6, 6 a, 7 and hence anycombination of probes 4, 4 a and/or the sample holder 3. So, the powersupply 8 is connected to a power source (not shown) and includesswitches for making connections between the power source and thedifferent wires 6, 6 a, 7. The power supply 8 can also variably andselectively limit the current that flows in any circuit formed by theprobes 4, 4 a and/or the sample holder, e.g. via the sample 5. In otherwords, the wires 6, 6 a and 7 can provide a potential difference and/orcurrent at either probe 4, 4 a to probe 4, 4 a or probe 4, 4 a to sampleholder 3. The voltage that the power supply 8 can provide issubstantially within the range around −50 V to around +50 V. Theelectric current that the power supply 8 can provide is substantiallywithin the range around 1×10⁻¹² A to around 1 A.

The probes 4, 4 a are capable of movement by translation in three-axes(x, y, z). Similarly, the sample holder 3 is capable of movement bytranslation in three-axes (x, y, z) and tilting and rotation. In otherembodiments different or additional types of movement can be providedfor both the probes 4, 4 a and the sample holder 3. The probes 4, 4 aand sample holder 3 can be moved with nanometre precision over a totalrange up to between around 10 μm to around 10 mm. In this embodiment,movement is achieved using piezoelectric actuators, although, in otherembodiments, other types of mechanical and electrical actuators can beused.

A control unit 9 is arranged to control the power supply 8 and movementof the probes 4, 4 a and sample holder 3 using the actuators. In thisembodiment, the controller 9 is a computer that runs software adapted tocarry out the methods described below and has an interface forcontrolling the power supply 8 and actuators. As well as controlling thepower supply 8, the controller 9 is able to monitor the potentialdifference and current generated by the power supply 8. Similarly, aswell as controlling movement of the probes 4, 4 a and the sample holder3, the controller 9 is able to control the SEM 1 and use image analysingsoftware to analyse the image generated by the SEM 1 and monitormovement of the probes 4, 4 a, sample holder 3 and even the individualcarbon nanotubes 10 a, as described in more detail below.

The carbon nanotubes 10 a can be prepared in a variety of ways and thesample 5 may therefore have one of several different forms. For example,the carbon nanotubes 10 a can be: attached to a Up that has been dippedinto a bundle of carbon nanotubes 10 a; embedded in a conducting polymersample which has been cleaved to expose the carbon nanotubes 10 a; orprepared using any other method that produces a sample 5 allowing thecarbon nanotubes 10 a to be brought into electrical contact between theprobes 4, 4 a or between one or both of the probes 4, 4 a and the sampleholder 3. Indeed, the invention is applicable to nanoscale wires otherthan carbon nanotubes, but these should be conductive and, if attachedto a substrate 10, it is useful if that too is conductive. However, theembodiments below are described in relation to a sample 5 comprisingcarbon nanotubes 10 a attached to catalytic particles forming asubstrate 10 from which the nanotubes 10 a have grown.

So, the apparatus can selectively move and apply voltages and currentsto the probe 4, 4 a or probes 4, 4 a and sample holder 3 under the SEM1. This allows an individual carbon nanotube 10 a to be selected, weldedto other structures such as the probe(s) 4, 4 a, substrate 5 or anothercarbon nanotube 10 a, or cut at a selected position along its length.These individual processes are described more fully below.

Selection of a Carbon Nanotube

Referring to FIG. 2, the sample 5 comprising a substrate 10 to whichseveral carbon nanotubes 10 a are attached is held in the sample holder3. The probe 4 can be moved relative to the sample holder 3 and hencerelative to the carbon nanotubes 10 a.

The controller 9 first focuses the SEM 1 in the plane of an end of atarget carbon nanotube 10 a distal to the substrate 10. The probe 4 isthen moved into the same plane as the end of the carbon nanotube 10 aand translated in that plane (the x, z plane in FIG. 1) toward thecarbon nanotube 10 a. Once this rough alignment has been carried out,the controller causes the power supply to apply a selection voltagesubstantially in the range of around 1 V to 2 V to the probe 4, with thesubstrate 10 and nanotube 10 a being held at ground, e.g. 0 V. At thesame time, the controller 9 causes the power supply 8 to limit thecurrent that is able to flow between the probe 4 and the sample holder3, e.g. via any of the nanotubes 10 a or the substrate 10, to below aselection current limit, e.g. substantially less than around 1 μA. Thepurpose of the selection voltage is to cause electrostatic attractionbetween the probe 4 and the target nanotube 10 a. The purpose of thecurrent limit is to ensure that, should the probe 4 contact any of thenanotubes 10 a, the current is insufficient to cause significant damageto the nanotube 10 a, e.g. by heating it enough to vaporise it.

The depth of field of the SEM 1 may be as deep as 500 nm. So, using thedepth of field of the SEM 1 may only allow the probe 4 and the targetcarbon nanotube 10 a to be positioned within around 500 nm of eachother. Once the probe is in the same depth of field as the target carbonnanotube 10 a, the controller 9 therefore causes the probe 4 to move indiscrete steps toward the nanotube 10 a. At the same time, thecontroller 9 monitors the position of the nanotube 10 a using the imageproduced by the SEM 1. When the gap between the probe 4 and the targetnanotube 10 a is small enough, electrostatic attraction will bend thenanotube 10 a toward the probe 4. This enables the approach of the probe4 to be carefully monitored.

As the nanotube 10 a is bent toward the probe 4, the controller 9monitors the current flowing between the probe 4 and the sample holder3. When the probe 4 is close enough and the nanotube 10 a bendssufficiently, the nanotube 10 a will contact the probe 4. Before contactis made, substantially no current flows between the probe 4 and thesample holder 3. However, when the nanotube 10 a and probe 4 makecontact, current flows between the probe 4 and the sample holder 3. Asthe controller 9 monitors the current, the controller 9 can identify theprecise moment that contact is made between the probe 4 and the nanotube10 a and, when contact is identified, the controller stops moving theprobe 4 relative to the substrate 10. The selection voltage applied tothe probe 4 can also be stopped or reduced. The target nanotube 10 a hasnow been selected.

Once the nanotube 10 a has been selected, the electrical properties(e.g. semi-conducting or metallic) of that particular nanotube 10 a aredetermined by applying a known voltage between the probe 4 and thesample holder 3 and measuring the current that flows. This can helpdetermine the quality of the nanotube 10 a and its usefulness for aparticular application. If a nanotube 10 a is not suitable, the contactcan be broken, e.g. by increasing the current to vaporise the nanotube10 a or just by withdrawing the probe 4, and an alternative nanotube 10a can be selected.

Welding a Nanotube to a Probe

Once a nanotube 10 a has been selected and the nanotube 10 a has beendeemed suitable, a current can be passed through the nanotube 10 a toheat the nanotube 10 a and, more importantly, its connection to theprobe 4. This welds the nanotube 10 a to the probe 4, improving theelectrical and mechanical contact between the nanotube 10 a and theprobe 4.

More specifically, before a nanotube 10 a is selected for welding, thecurrent at which the nanotubes 10 a of a particular sample 5 fail, e.g.by over-heating and vaporisation, is determined. In this embodiment,this is achieved by the controller 9 selecting a nanotube 10 a of thesample 5 and, once contact has been established, causing the powersupply 8 to gradually increase the current flowing through the nanotube10 a until it fails. When it fails, the current drops sharply to zero.The controller 9 monitors the current and determines the maximum currentflowing though the nanotube 10 a, which is usually just before thenanotube 10 a fails or becomes structurally damaged. This is called thefailure current. The process is usually repeated for two or morenanotubes 10 a and a welding current limit is set below a typical (e.g.the lowest or average) determined failure current. Of course, oncesignificant experience has been gained with a particular type or batchof samples 5, the welding current limit can be reliably determined andit is not necessary to set a new limit for every sample 5.

When a nanotube 10 a is selected, as described above, the small currentthat flows at the moment that contact is made heats the nanotube 10 a inthe area of the contact. More specifically, as the contact between thenanotube 10 a and the probe 4 is initially electrically poor, e.g. hashigh resistance, in comparison to the rest of the nanotube 10 a, andindeed the probe 4 and substrate 10, this region is heated to a highertemperature than the rest of nanotube 10 a. This results in a smallamount of diffusion of material between the nanotube 10 a and the probe4 at the contact. However, as the current limit during selection is verylow, the heating and diffusion at the contact is minimal and theelectrical connection remains poor.

So, once the nanotube 10 a has been selected and contact been made, thecontact is welded to improve the connection. This is achieved byincreasing or “ramping” the voltage across the contact, e.g. between theprobe 4 and the sample holder 3, in a controlled manner and allowing thecurrent to rise to the welding current limit. In one embodiment, thecontroller 9 causes the power supply 8 to increase the current and tohold it at a steady level for a predetermined duration, which can besubstantially between around 1 s and 100 s. The current heats thecontact between the nanotube 10 a and the probe 4 resulting in furtherdiffusion of material between the nanotube 10 a and the probe 4 (e.g.“inter-diffusion”). The weld that is formed therefore has improvedelectrical and mechanical properties.

In another embodiment, the controller 9 causes the power supply 8 torepeatedly apply a voltage across the contact, e.g. between the probe 4and the sample holder 3. More specifically, the voltage is increased andthen decreased over a short period of time on more than one separateoccasion. By monitoring the current as the voltage is increased anddecreased, it is possible to see the improvement in quality of theelectrical connection, e.g. as its resistance is lowered. In otherwords, whilst the flow of current causes heating that improves thecontact, the resistance across the contact changes during application ofthe voltage. So, referring to FIG. 3, a plot of current to voltage showsa different curve as the voltage is increased (e.g. curve A) incomparison to when it is decreased (e.g. curve B) for each voltageapplication (11 a-e). However, when there is no longer any improvementof the electric connection of the contact, the resistance does notchange significantly and the curve as the voltage is increased isvirtually the same as the curve as the voltage is decreased (see, e.g.curve lie in FIG. 3) So, the first step is for the controller 9 toestablish that contact has been made by causing the power supply 8 toapply a low voltage, e.g. ±1 V, across the contact and detecting whetheror not any current, e.g. around a few nA, flows across the contact. If acurrent flows, the controller 9 determines that contact between theprobe 4 and the nanotube 10 a has been made. This is effectively thesame step as confirming contact has been made with a target nanotube 10a during selection, as described above. If no current flows, the processof selecting a nanotube 10 a is repeated.

Next, the controller 9 causes the power supply to increase the voltagebetween the probe 4 and the sample holder 3. In this embodiment, thecontroller 9 increases the voltage in steps, e.g. of around 0.1 V. Thecurrent is held at each step, e.g. for around a few ms or more. Eachtime the voltage is increased, the current is measured.

While the voltage Is increased, the controller 9 causes the power supply8 to limit the current to the welding current limit. Typically, thislimit is no greater than around 1 μA. Likewise the controller 9 limitsthe voltage to a welding voltage limit. The welding voltage limit istypically around a few volts. So, the controller 9 causes power supplyto stop increasing the voltage when either the welding current limit isreached or the welding voltage limit is reached. When the current limitor the voltage limit is reached, the controller 9 causes the powersupply 8 to decrease the voltage in steps, e.g. of around 0.1 V, back to0 V. Again, each time the voltage is decreased, the current is measured.This increase and decrease of voltage can be referred to as aconditioning cycle.

Following or during each conditioning cycle, the controller 9 determinesthe quality of the contact. This is achieved by the controller 9comparing the current measurements as the voltage is/was increasedduring the conditioning cycle with respective current measurements asthe voltage is/was decreased during the conditioning cycle. Comparisonat one selected voltage is sufficient. However, to improve accuracy,several comparisons are made or the current-voltage curve as the voltageis/was increased is compared to the current voltage curve as the voltageis/was decreased. As can be seen in FIG. 3, if the contact is poor, thensignificant differences are seen on the increasing and decreasingcurves, e.g. there is hysteresis. However, if the contact is good, theresistance of the contact is not improved over the conditioning cycleand there is no substantial difference on the increasing and decreasingdata curves.

So, if there is more than a pre-set difference, e.g. 1%, between thecurrent at respective (or coincident) voltages during increasing anddecreasing phases of the cycle, the controller 9 performs anotherconditioning cycle. Alternatively, if the controller 9 determines thatthere is no or less than the pre-set difference between the two currentsor sets of currents, then it determines that the electrical connectionof the contact is good. The controller 9 does not then perform anyfurther conditioning cycles.

If the controller 9 determines that another conditioning cycle should beperformed, it also determines whether the voltage limit was reached orwhether the current limit was reached to cause it to stop increasing thevoltage in the previous conditioning cycle. If the voltage limit wasreached, the voltage limit is increased, e.g. by around 1 V. If thecurrent limit was reached, the current limit is increased, e.g. byaround 1 μA. The next conditioning cycle is then performed using thehigher voltage or current limit, with the result that a higher currentis passed across the contact.

The controller 9 continues to perform conditioning cycles in this manneruntil it determines that the quality of the contact is no longerimproving, e.g. that there is less than say a 1% difference in thecurrent at the respective (or coincident) voltage(s) during theincreasing and decreasing phases of the cycle. This allows improvementto the contact between the nanotube 10 a and the probe 4 while ensuringthat the current flow is under strict control and excessive currentheating does not damage the nanotube 10 a. Furthermore, the controlledapplication of the voltage enables a conditioned weld to be establishedquickly and safely.

Conditioning a Nanotube

In a manner similar to that used for conditioning welds it is possibleto condition an individual nanotube 10 a. For example, when nanotubes 10a are grown at low temperature by catalytic methods it is known thatthey often contain curls and kinks. It is possible to straighten thesecurls and kinks and perform other types of conditioning using thepresent invention.

When a nanotube 10 a, which is connected to its substrate 10 in thesample holder 3, has been welded to the probe 4 using the above method,it is securely held at each end and there is a good electricalconnection at each end. Relatively high currents can therefore be passedalong the nanotube 10 a without the probe 4/nanotube 10 a or nanotube 10a/substrate 10 contact being damaged. Furthermore, the probe 4 can bemoved relative to the sample holder 3 to exert mechanical strain on thenanotube 10 a.

For example, if the nanotube 10 a is curved, once it has been welded tothe probe 4, the controller 9 moves the probe 4 away from the substrate10. This straightens the nanotube 10 a. The controller 9 then causes thepower supply 8 to pass current through the nanotube 10 a to heat thenanotube 10 a for a fixed duration. This anneals the nanotube 10 a, sothat its structure becomes straighter. Indeed, the controller 9 can passcurrent though the nanotube 10 a to cause heating at the same time asprogressively moving the probe 4 away from the sample holder 3. Thus, asignificant amount of straightening can be achieved.

In another embodiment, the controller 9 moves the probe 4 to inducecurves in a straight nanotube 10 a. This can cause the nanotube 10 a todevelop particular electrical characteristics, such as quantum dots.

In another embodiment, the controller 9 heats the nanotube by varyingthe applied voltage in a way similar to during a conditioning cycle fora weld, as described above. In other words, the controller 9 increasesand decreases the voltage whilst monitoring the current and repeats thisuntil it determines that the electrical characteristics of the nanotube10 a are no longer improving. Thus, reliable improvements in theelectrical characteristics of the whole nanotube 10 a can be achieved.Of course, the probe 4 can also be moved before or during application ofthe voltage(s) to straighten or bend the nanotube as desired.

So, it is possible to both improve the electrical behaviour orcharacteristics of nanotube 10 a without straightening or bending thenanotube 10 a, or to move the probe 4 relative to the sample holder 3 toexert strain on the nanotube and improve the electrical behaviour whilestraightening or bending the nanotube 10 a to some extent.

Cutting a Nanotube

Once a nanotube 10 a has been welded to the probe 4, it is useful to beable to either cut the nanotube 10 a somewhere along its length or atthe end at which it is attached to the substrate 10. To achieve this,the second probe 4 a, which is able to move independently of the firstprobe 4, is used.

Referring to FIG. 4, the controller 9 moves the second probe 4 a towardthe nanotube 10 a at a point at which it is desired to cut the nanotube10 a. The controller 9 then causes the power supply to apply theselection voltage, e.g. around 1 V to 2 V, to the second probe 4 a,whilst the first probe 4 and the substrate are held at ground voltage,e.g. 0V. This defines the point at which it is desired to cut thenanotube 10 a . So, the selection process is effectively repeated, usingthe second probe 4 a and the nanotube 10 a already welded to the firstprobe 4.

If it is desired to keep the part of the nanotube 10 a welded to thefirst probe 4, once contact has been established between the secondprobe 4 a and the nanotube 10 a, the controller 9 causes the powersupply 8 to apply a voltage between second probe 4 a and the sampleholder 3 that causes the current in the portion of the nanotube 10 abetween the second probe 4 a and the substrate 10 to exceed the failurecurrent (e.g. apply a current usually around tens of μA to hundreds ofμA). No current is passed though the portion of the nanotube 10 abetween the second probe 4 a and the first probe 4. So, the portion ofthe nanotube 10 a between the second probe 4 a and the substrate 10vaporises, leaving the portion of the nanotube 10 a between the secondprobe 4 a and the first probe 4 intact and still welded to the firstprobe 4 a. By appropriate selection of the point at which the secondprobe 4 a contacts the nanotube 10 a, the nanotube 10 a can therefore becut at any desired point along its length. The nanotube 10 a can then bemoved freely, e.g. to another region of the sample 5 or to anothersubstrate 10.

In another embodiment, a small gap can be left between the second probe4 a and the nanotube 10 a. An alternating voltage is then appliedbetween the second probe 4 a and the nanotube 10 a, which causes a smallportion of the nanotube 10 a nearest to the second probe 4 a tovaporise. This results in two portions of the nanotube 10 a remaining,one welded to the first probe. 4 and the other attached to substrate 10,as shown in FIG. 4.

As with the welding the nanotubes 10 a and conditioning them, thisprocess can be controlled by the controller 9. In the simplest case, theprobe 4 a can be positioned manually and the controller 9 used tocontrol the voltage and current flow at each tip and substraterespectively. Alternatively, the controller 9 can use the image from theSEM 1 to position the probes 4, 4 a and the whole cutting process can beautomated. The controller 9 can offer significant improvements in boththe speed and repeatability of the cutting and shortening processes.

Welding a Nanotube to another Structure

The welding process described above can be used equally well to weld ananotube 10 a to structures other than the probes 4, 4 a. For example, ananotube 10 a can be welded to other nanotubes 10 a (see, e.g. FIG. 5)or to other structures or substrates (not shown). Usually, the nanotube10 a is first welded to the first probe 4 and cut away from thesubstrate 10 using the above welding and cutting processes. The nanotube10 a then, in effect, becomes an extension of the probe 4. This meansthat it can be moved to touch other nanotubes 10 a or substrates 10 andbe welded to them using the welding process described above. Indeed, itis possible to weld nanotubes 10 a end-to-end to create a longernanotube from dissimilar nanotubes, and also weld nanotubes to the sidesof other tubes to create nanotubes in ‘T’ formations, as shown in FIG.5. Hence, nanotube devices with more than two terminals can be created.Single nanotubes or welded nanotube combinations can then be welded toother suitable structures or substrates, again using the methodsdescribed above. The only requirement is that the other structures orsubstrates are electrically conductive and can be connected to the powersupply 8.

Using the techniques described above, it is possible to constructelectronic devices based on carbon nanotubes 10 a of considerablygreater complexity than has been previously demonstrated. Similar ordissimilar nanotubes 10 a can be welded together in a large variety ofconfigurations, and to suitable substrates 10 that can be connected inturn to other devices, to produce carbon nanotube electronic devices(and related structures) selectively and with a high degree of success.Examples of other useful structures include high-aspect ratio extensionsto scanning probe microscope tips, mechanical actuators in microelectrical machine system (MEMS) devices and sensors.

As the whole, the above nanotube selection, welding and cuttingprocesses are based on the careful control of voltage and current flowand movement of the probes 4, 4 a relative to the sample holder 3. Thecontroller 9 uses the power supply 8 to control and monitor current andvoltage. It also uses the SEM image and feedback from the actuators toestablish the positions in three dimensional space of the probes 4, 4 a,nanotubes 10 a and substrate 10. The processes can therefore be fully orpartially automated as desired.

The described embodiments of the invention are only examples of how theinvention may be implemented. Modifications, variations and changes tothe described embodiments will occur to those having appropriate skillsand knowledge. These modifications, variations and changes may be madewithout departure from the spirit and scope of the invention defined inthe claims and its equivalents.

1. A method of welding a nanoscale wire to a structure, the methodcomprising: positioning the nanoscale wire and the structure in contactwith one another; and applying a voltage across the contact so that acurrent flows through the contact and welds the wire to the structure.2. The method of claim 1, further comprising limiting the current thatflows through the contact during welding.
 3. The method of claim 2,wherein the current is limited to a current threshold level lower thanan estimated typical current at which the nanoscale wire fails or isstructurally damaged.
 4. The method of any of claim 1, wherein thecurrent threshold level is less than around 10 μA.
 5. The method of any,one of claim 1, wherein the voltage is less than around 5V.
 6. Themethod of claim 1, comprising applying the voltage across the contactduring plural separate intervals.
 7. The method of claim 1, comprisingmonitoring the current during application of the voltage.
 8. The methodof claim 1, comprising comparing the current when a known voltage isapplied with the current when that voltage is applied again to monitorthe change in resistance of the contact.
 9. The method of claim 8,comprising continuing to apply the voltage(s) across the contact untilthere is no substantial difference in the compared currents.
 10. Themethod of claim 1, wherein the structure is a nanoscale probe.
 11. Themethod of claim 1, wherein the structure is another nanoscale wire. 12.(canceled)
 13. A method of annealing a nanoscale wire, the methodcomprising welding a probe to the wire and passing a current along thewire via the probe sufficient to heat the wire and cause annealing. 14.The method of claim 13, wherein the probe is movable and the methodcomprises moving the probe to exert strain on the wire during annealing.15. The method of claim 14, comprising exerting strain on the wire bybending the wire.
 16. The method of claim 14, comprising exerting strainon the wire by straightening the wire.
 17. (canceled)
 18. A method ofcutting a nanoscale wire, the method comprising: positioning a cuttingprobe at a position along the length of the wire intermediate twopositions at which the wire is held; and applying an electricalpotential between the cutting probe and the wire to cut the wire at theposition along the length of the wire.
 19. The method of claim 18,wherein the cutting probe is positioned to touch the wire at theposition along the length of the wire and the electrical potential isapplied only between the cutting probe and one of the two positions atwhich the wire is held.
 20. The method of claim 18, wherein the appliedpotential is controlled to pass a current exceeding an or the estimatedcurrent at which the nanowire fails or is structurally damaged.
 21. Themethod of claim 18, wherein the cutting probe is positioned so that itis closest to the wire at the position along the length of the wire, butslightly spaced away from the wire.
 22. The method of claim 21, whereinthe applied electrical potential is alternated.
 23. The method of claim1, wherein the nanoscale wire is a carbon nanotube.
 24. A nanoscalestructure produced using the method of claim
 1. 25. A nanoscalestructure comprising two or more nanoscale wires welded together usingthe method of claim
 1. 26. A nanoscale structure comprising a nanoscalewire annealed using the method of claim
 13. 27. (canceled)
 28. Anapparatus for welding a nanoscale wire to a substrate, the apparatuscomprising: a manipulator for positioning the nanoscale wire and thestructure in contact with one another; and a controller for applying avoltage across the contact so that current flows through the contactduring welding.
 29. The apparatus of claim 28, wherein the controllerlimits the current that flows through the contact during welding. 30.The apparatus of claim 29, wherein the controller limits the current toa threshold level lower than an estimated typical current at which thenanoscale wire fails or is structurally damaged.
 31. The apparatus ofclaim 29, wherein the current threshold level is less than around 10 μA.32. The apparatus of claim 28, wherein the voltage is less than around5V.
 33. The apparatus of claim 28, wherein the controller applies thevoltage during plural separate intervals.
 34. The apparatus of claim 28,wherein the controller monitors the current during application of thevoltage.
 35. The apparatus of claim 28, wherein the controller comparesthe current when a known voltage is applied with the current when thatvoltage is applied again to monitor the change in resistance of thecontact.
 36. The apparatus of claim 35, wherein the controller continuesto apply the voltage(s) across the contact until there is no substantialdifference in the compared currents.
 37. The apparatus of claim 28,wherein the structure is a probe for manipulating a nanoscale wire. 38.The apparatus of claim 28, wherein the structure is another nanoscalewire.
 39. (canceled)
 40. An apparatus for annealing a nanoscale wire,the apparatus comprising means for welding a probe to the wire and acontroller for passing a current along the wire via the probe sufficientto heat the wire and cause annealing.
 41. The apparatus of claim 40,comprising a manipulator for moving the probe to exert strain on thewire during annealing.
 42. The apparatus of claim 40, wherein themanipulator moves the probe to exert strain on the wire by bending thewire.
 43. The apparatus of claim 40, wherein the manipulator moves theprobe to exert strain on the wire by straightening the wire. 44.(canceled)
 45. An apparatus for cutting a nanoscale wire, the methodcomprising: a manipulator for positioning a cutting probe at a positionalong the length of the wire intermediate two positions at which thewire is held; and a controller for applying an electrical potentialbetween the cutting probe and the wire to cut the wire at the positionalong the length of the wire.
 46. The apparatus of claim 45, wherein thecutting probe is positioned to touch the wire at the position along thelength of the wire and the controller applies the electrical potentialonly between the cutting probe and one of the two positions at which thewire is held.
 47. The apparatus of claim 46, wherein the controllerapplies the electric potential so that a current is passed that exceedsa or the estimated typical current at which the nanoscale wire fails oris structurally damaged.
 48. The apparatus of claim 45, wherein themanipulator positions the probe so that it is closest to the wire at theposition along the length of the wire, but slightly spaced away from thewire.
 49. The apparatus of claim 48, wherein the applied electricalpotential is alternated.
 50. The apparatus of claim 28, wherein thenanoscale wire is a carbon nanotube.
 51. Computer software adapted tocarry out the method of claim 1 when processed by a processor.
 52. Thecomputer software of claim 51 carried by a data carrier.
 53. (canceled)54. (canceled)
 55. The method of claim 13, wherein the nanoscale wire isa carbon nanotube(s).
 56. The method of claim 18, wherein the nanoscalewire is a carbon nanotube.
 57. The apparatus of claim 40, wherein thenanoscale wire is a carbon nanotube.
 58. The apparatus of claim 40,wherein the nanoscale wire is a carbon nanotube.
 59. Computer softwareadapted to carry out the method of claim 13 when processed by aprocessor.
 60. Computer software adapted to carry out the method ofclaim 18 when processed by a processor.